Okay since IMVU will be taking away all posts covering this question, I have decided to make a public room picture layout that covers all public rooms. This here is my first shot at a layout. I have dug through many of the old IMVU forum posts covering this question, as well as testing myself and have come up with these dimensions.

Public room info. cards only show the top 332x147 of your total image. For featured rooms, they show the full 332x281 image. Also note that on featured rooms they have a hazed out bar at the bottom, starting at 218, where the room name and owners name appear along with the rooms rating. I have included the room rating stars, as well as user count in case someone wishes to shade those areas in their final images. (side note, when receiving an invite to a public room, the full image will show to the person who receives the invite.)

I know as IMVU changes, things like this can change as well. If you find my dimensions are off at all, please let me know and I will be sure to fix it.